Innovative approaches are urgently needed to improve behavioral treatment for weight loss. The weight regain that is so common after treatment may be a result of an environment that makes it challenging to adhere, long-term, to a dietary and physical activity regimen. This study was designed to test, via a 12-week open trial, the preliminary feasibility, acceptability, effectiveness, and possible mechanisms of action of a behavioral treatment that was modified to incorporate acceptance-based therapy components designed to (a) bolster participants' commitment to behavior change, (b) build distress-tolerance skills, and (c) promote mindful awareness of eating behaviors and goals. Participants (n=29) were overweight or obese women. Among completers (n = 19; 34% attrition), weight loss averaged 6.6% of body weight at posttreatment and 9.6% at 6-month follow-up (n = 14; 52% attrition). Intention-to-treat weight losses were 4.5% at posttreatment and 6.6% at 6-month follow-up. Psychological variables targeted by the intervention (e.g., cognitive restraint, disinhibition, urge-related eating behavior, emotional eating, eating-related experiential acceptance, mindfulness and motivation) changed in the expected directions, and many of these changes were consistent with decreases in weight loss. Moreover, despite the limitations of the single-group design, this pilot study demonstrated the preliminary feasibility, acceptability, and effectiveness of a novel, acceptance-based behavioral treatment for obesity. One potential implication is that behaviorally based weight loss interventions might be improved by overlaying an acceptance-based framework. (Contains 4 tables.)